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1. Technical Field
The present invention relates to a method of and apparatus for measuring parameters of an electronic system by reference to an input series of data samples received and processed at a first data rate so as to produce in real-time at least first and second time-varying series of measurements for a given parameter. The invention may be applied in the measurement of timing errors in digital transmission systems, for example, standardised measurement known as Timing Deviation (TDEV) in Synchronous Digital Hierarchy (SDH) digital transmission systems in accordance with specifications as set out by the ITU-T (xe2x80x9cITUxe2x80x9d stands for International Telecommunications Union).
2. Background Art
Modern telecommunications networks demand a high degree of synchronisation between network transmission elements. For all network transmission elements in SDH architectures timing is critical. However, as will be explained later, phase variations in the reference clock frequencies governing synchronous network elements may introduce errors at various stages in the network.
One measure of timing errors in synchronous digital transmission systems is known as the timing deviation and is derived from an ensemble of timing error samples. This is a measure of the time variation of a signal and can also provide information oh the noise signal. In SDH systems, the timing error samples are referred to as xe2x80x9cTime Interval Errorxe2x80x9d or TIE samples, and a standardised timing deviation measure referred to as TDEV is defined. TDEV values, together with other parameters, are used to evaluate the performance of equipment and systems, often to diagnose a fault which has developed and which impairs customer service.
Unfortunately, implementing directly the definition of TDEV (or similar parameters) provided by the standards bodies does not permit a real-time display of the results. In particular, TDEV is generally required to be measured for on a set of different times (observation intervals), to reveal information about the time varying behaviour of the signal, and aid in diagnosis of faults. The observation intervals typically range from one second up to a day or more. To obtain the results for such intervals conventionally requires a large quantity of data to be collected and, in principle, even for the shortest observation interval, TDEV cannot be calculated until the entire data set has been gathered. This is clearly inconvenient but if, to obtain a quicker result, TDEV for the observation intervals is calculated using a partial set of data, the calculations performed must be performed again as more data becomes available. One proposal which provides intermediate results at an early stage is JP-A-10178420 (Anritsu) (Derwent Abstract accession number 98-424471/199836 refers).
An alternative expression of TDEV can be formulated, in the frequency domain. From this, it becomes possible to implement the calculations of TDEV for the different observation intervals as a bank of filters, each with its own band-pass characteristics. This offers a real-time implementation, in which estimates of the measurement for the shorter observation intervals become more quickly available. Short term problems may reveal themselves as the early results are replaced with new data. Results for longer observation intervals will become available as time progresses. One system which purports to offer real-time TDEV measurement on this basis is Flexacom Plus, advertised by ICT Electronics on the World Wide Web.
A problem which remains with the filter implementation is the large amount of data storage and computation needed to obtain the measured values of TDEV or the like, particularly for the longer observation intervals. The ITU-T specifies a minimum sample rate of 30 Hz, while samples covering at least three times the observation interval are generally required to obtain one measurement.
The invention aims to permit real-time calculation of a set of measurements such as TDEV for a range of observation intervals, while reducing the computational burden involved.
The invention in a first aspect provides a method of measuring parameters of an electronic system by reference to an input series of data samples (TIE) received and processed at a first data rate so as to produce in real-time at least first and second time-varying series of measurements for a given parameter (TDEV), each series of measurements resulting from the performance of a respective first and second process by digital data processing, the first and second series of measurements representing said parameter of the system nominally with regard to respective first and second observation intervals , the second observation interval being longer than the first, wherein:
the first process is implemented in real time by performing a first low pass filter process on said input series of data samples so as to produce a first low pass filtered series of data samples, and performing a first further process on said first low pass filtered series of samples to obtain the first series of measurements;
a first reduced rate series of samples is extracted from said first low pass filtered series of data samples; and
the second process is performed in real time using said first reduced rate series of samples.
In the case of TDEV calculation, each of the first process and the second process comprises an appropriate band-pass filter function, followed by an RMS power estimation function. By performing the low-pass element of this separately, a filtered sample set is available, which can be used to provide a lower data rate input for the second band-pass filter. The reduction in the overall amount of calculation, when several TDEV intervals are being calculated at once, means that real-time calculation may be carried out economically [low cost Digital Signal Processor (DSP) solution built into a portable test instrument].
In a first embodiment of the invention a series of band-pass filter stages is implemented in parallel, one for each observation interval. Each stage (ignoring the last stage) is divided into low-pass and high-pass filters, and the low-pass filtered samples are used as the input to the subsequent stage. Since the input samples for each stage are pre-filtered in this way, the data set needed to estimate TDEV at different time intervals can be reduced without loss of information.
Various arrangements are possible, which can be chosen according to the exact observation intervals required, and the economics of a chosen implementation. For example, to implement the low-pass and high-pass functions separately is likely to be more costly than direct implementation of a band-pass function. Accordingly, in a second embodiment of the invention, the samples are not separately low-pass filtered and reduced at every stage, but only at certain key stages.
The invention further provides an apparatus for measuring parameters of an electronic system by reference to an input series of data samples received and processed at a first data rate so as to produce in real-time at least first and second time-varying series of measurements for a given parameter, each series of measurements resulting from the performance of a respective first and second process by digital data processing, the first and second series of measurements representing said parameter of the system nominally with regard to respective first and second observation intervals, the second observation interval being longer than the first, the apparatus comprising:
means for performing said first process in real-time by (i) performing a first low pass filter process on said input series data samples so as to produce a first low pass filtered series of data samples, and (ii) performing a first further process on said first low pass filtered series of samples to obtain the first series of measurements;
means for extracting a first reduced rate series of samples is extracted from said first low pass filtered series of data samples; and
means for performing said second process in real time using said first reduced rate series of samples.
The first and second processes can conveniently be implemented in a single digital signal processor chip, although of course hard-wired filter arrangements could be used instead.
Further optional features are set forth in the dependent claims. These and other features, together with their advantages will be apparent to the skilled reader from the description of specific embodiments which follows.
It will be understood that xe2x80x9creal timexe2x80x9d in this context does not imply that results are available without delay, or must be strictly synchronised with the flow of input samples. xe2x80x9cReal timexe2x80x9d in this context signifies merely that input samples can be processed, on average, substantially at the rate at which the input samples are generated.